As a method of electroreduction of a disulfide compound, a method using an alloy consisting of two or more kinds of particular metals as a cathode is known (patent document 1). In addition, as a method of producing L-cysteine by electroreduction of L-cystine, which is one kind of disulfide compound, a method using a cation exchange membrane as a separating membrane, and an L-cystine solution acidified by adding a mineral acid such as hydrochloric acid and the like to a cathode side electrolytic cell is known (patent document 2). As a method of producing reduced glutathione by electroreduction of oxidized glutathione, a method using an aqueous oxidized glutathione solution acidified by adding a mineral acid such as concentrated hydrochloric acid and the like to a cathode side electrolytic cell is also known (patent document 3).
However, the above-mentioned electroreduction methods of a disulfide compound require use of an expensive electrode. When oxidized glutathione is electroreduced under L-cystine electroreduction conditions described in patent document 2, reduced glutathione cannot be produced in a sufficient yield, since it is unstable under strong acidity and high temperature, as compared to L-cysteine. The method described in patent document 3 also uses, similar to the method described in patent document 2, aqueous oxidized glutathione solution strongly acidified to pH 0.6-1.0 by adding a mineral acid, and therefore, it is associated with the problems of corrosion of cathode and decomposition of reduced glutathione. To minimize the decomposition of reduced glutathione under strong acidity, it is necessary to lower the electric current density. In this case, however, reduction efficiency per electrode area decreases, and therefore, efficient production of reduced glutathione cannot be achieved unless the electrode area is increased instead. That is, production of reduced glutathione in an industrial scale by electroreduction of aqueous oxidized glutathione solution strongly acidified with a mineral acid requires huge electroreduction facility corresponding to the sizes of ion exchange membrane and electrode, as well as a special electrode capable of resisting corrosion. Therefore, a production method of reduced glutathione by conventional electroreduction is not entirely a realistic method in terms of production efficiency and facility.